An axial flow rotary machine, such as a gas turbine engine for an aircraft, has a compression section, a combustion section, and a turbine section. An annular flow path for working medium gases extends axially through the sections of the engine. The gases are compressed in the compression section to raise their temperature and pressure. Fuel is burned with the working medium gases in the combustion section to further increase the temperature of the hot, pressurized gases. The hot, working medium gases are expanded through the turbine section to produce thrust and to extract energy as rotational work from the gases. The rotational work is transferred to the compression section to raise the pressure of the incoming gases.
The compression section and turbine section have a rotor which extends axially through the engine. The rotor is disposed about an axis of rotation Ar. The rotor includes arrays of rotor blades which transfer rotational work between the rotor and the hot working medium gases. Each rotor blade has an airfoil for this purpose which extends outwardly across the working medium flow path. The working medium gases are directed through the airfoils. The airfoils in the turbine section receive energy from the working medium gases and drive the rotor at high speeds about an axis of rotation. The airfoils in the compression section transfer this energy to the working medium gases to compress the gases as the airfoils are driven about the axis of rotation by the rotor.
The engine includes a stator disposed about the rotor. The stator has an outer case and arrays of stator vanes which extend inwardly across the working medium flowpath. The outer case extends circumferentially about the working medium flow path to bound the flow path. The stator has seal elements for this purpose, such as a circumferentially extending seal member which is disposed radially about the rotor blades. The seal member is in close proximity to the tips of the rotor blades to form a seal that blocks the leakage of working medium gases from the flowpath.
The arrays of stator vanes are disposed upstream of the arrays of rotor blades in both the compression section and turbine section. The stator vanes each have an airfoil for guiding the working medium gases to the rotor blades as the gases are flowed along the flow path. The airfoils of the stator vanes and the rotor blades are designed to receive, interact with and discharge the working medium gases as the gases are flowed through the engine.
As a result, the airfoils are bathed in hot working medium gases during operation of the rotor blades causing thermal stresses in the airfoils. These thermal stresses affect the structural integrity and fatigue life of the airfoil. In addition, rotational forces acting on the rotor blade cause additional stresses in the rotor blade, such as mechanical stresses, as the rotor blade is driven about the axis of rotation.
One way to reduce stresses in rotor blades downstream of the combustion section in the high pressure turbine is to cool the rotor blades by flowing cooling fluid through the airfoil. The cooling fluid removes heat from the airfoil decreasing thermal stresses and avoiding unacceptably high temperatures for the material used for the walls of the airfoil. Each rotor blade has one or more openings at its inner end for receiving the cooling air.
One source of cooling fluid is working medium gases from the compression section (pressurized air). The fluid bypasses the combustion process and is at a much lower temperature than the working fluid in the turbine section. The cooling fluid is flowed through and around various structures within the turbine section.
The use of such a cooling fluid has a negative impact on the aerodynamic efficiency of the gas turbine engine. This occurs because the compressed cooling fluid bypass the combustion section where energy is added to the fluid and enters the flowpath with little transfer of useful energy from the compressed fluid to the turbine stages. The loss of efficiency is balanced by increased durability of the parts and higher combustion temperatures that increase cycle efficiency. This balancing emphasizes the need to use efficiently the cooling fluid drawn from the compressor section.
The turbine airfoils have complex internal passages for receiving and discharging the cooling fluid. As the cooling fluid passes through these passages, heat is transferred from internal surfaces to the cooling fluid. The surfaces include surfaces on heat transfer members, such as trip strips and pedestals, to increase heat transfer between the cooling fluid and the turbine airfoil. The cooling fluid then exits into the flow path through cooling holes distributed about the airfoil.
An example of such a rotor blade is shown in U.S. Pat. No. 4,474,532 entitled "Coolable Airfoil For a Rotary Machine", issued to Pazder and assigned to the assignee of this application. Another example of a coolable rotor blade is shown in U.S. Pat. No. 4,278,400 issued to Yamarik and Levengood entitled "Coolable Rotor Blade" and assigned to the assignee of this application. Each of these rotor blades is provided with a plurality of cooling air passages on the interior of the blade. Cooling air is flowed through the passages to the rearmost portion of the rotor blade, commonly referred to as the trailing edge, from whence the cooling air is exhausted into the working medium flowpath.
U.S. Pat. No. 5,368,441 issued to Sylvestro entitled "Turbine Airfoil Including Diffusing Trailing Edge Pedestals" has a plurality of flow dividers (teardrop shaped pedestals) in the trailing edge region of the blade. The pedestals place the interior of the airfoil of the rotor blade in flow communication with the exterior of the blade. The flow of cooling fluid directly cools the interior of the blade by impingement on the internal surfaces and convection as the flow proceeds through cooling channels. The trailing edge is cut back on the pressure side to uncover a diffusing region at the end of the flow dividers. A channel is formed between the flow dividers to diffuse cooling air over the suction surface of the airfoil and provide a film of cooling air over the suction wall. The film cooling is effective but its effectiveness decreases if the flow separates from the walls of the diffusing section and turbulently mixes with the hot working medium gases.
Other constructions have a plurality of pedestals in the trailing edge region to increase heat transfer. Increasingly intricate constructions have been formed by casting the airfoil with a lost wax process. These constructions may be formed upstream of flow dividers at the trailing edge of the airfoil but in close proximity to increase heat transfer to the cooling fluid. These include constructions in which pairs of spanwisely extending ribs in the trailing edge region have a plurality of spanwisely spaced orifices for directing cooling air on adjacent internal structure. Impingement cooling results, improving heat transfer in the trailing edge of the airfoil. The ribs in such constructions are relatively small in the chordwise direction but relatively long in the spanwise direction. The small orifices when coupled with the need during the casting process to have wax where metal will form and core material (slurry) only where the small orifices are formed in the casting results in only small amounts of core material being disposed in the rib location of the casting core.
The ribs may cause difficulties during the casting process. During the casting process of such an airfoil, the ceramic core (which defines the openings and structure in the trailing edge region) is disposed on the interior of an airfoil shaped mold. Molten metal is poured around the core, rushing into the mold during the pouring process. The molten metal fills openings in the core to form solid structure and flows around the solid ceramic core material to form holes, such as the orifices in the ribs. As the molten metal enters the structure, portions of the core in the trailing edge region where the heat transfer elements are intricate and delicately formed, may collapse resulting in an unusable casting. Accordingly, the complexity of the heat transfer members is balanced by the need to cast these features.
The above notwithstanding, scientists and engineers working under the direction of Applicants Assignee have sought to develop cooling constructions for the trailing edge region of a rotor blade which have acceptable castability during manufacture and have acceptable levels of blade stress and metal temperature under operative conditions.